The answer will surprise you. And as the household breadwinner stakes continue to rise, The Richer Sex author Liza Mundy shares the key professions where it pays to be a mother, and why.

For the first time in modern history, mothers in some key professions may enjoy the same earnings boost that fathers have long enjoyed. At the 2011 conference of the Population Association of America, one of the world’s leading assemblies of demographers, sociologist Anne McDaniel reported on new data she and Claudia Buchmann uncovered, which showed that in some fields mothers earn more than women without children. It’s difficult to gauge from the data why this might be, but any working mother can tell you that people are very productive when they know they have to squeeze in a day’s work before child care closes at 6 p.m.

In an accompanying paper, the authors underlined just how much women’s aspirations have changed. In 1980, the most common professional job for women was teaching. Now it’s business. And here is what is most interesting: often there is no clear wage penalty for having children. In the life sciences, medicine, and law, women with young children earn more than women without children, when they controlled for hours worked. In medicine, for example, women with young children earn 9 percent more than childless women. Being “married with young children is associated with higher earnings than being single without children in math and physical science, engineering and computer sciences, life sciences, medicine, law, and business,” they noted. Controlling for education, age, and work hours, “women with young children earn more in most elite occupations than women without children.”

This was not a result she had expected. But it makes sense, particularly when you take into account single mothers whose families depend solely on their wages. Four out of 10 babies in the United States are now born to unmarried mothers. Single mothers have extremely high labor force participation rates. I interviewed a number of mothers who said that having a child made them more productive and serious. “When I got pregnant, I couldn’t party, so I focused more on school,” said Diandra Prieto, a married dietitian who had her first child as an undergraduate. Rather than driving her off course, becoming a mother made her more industrious. “I excelled pretty easily,” she said.

As women envision themselves as breadwinners, having a child will be motivation to work harder—not drop out. When I asked McDaniel whether mothers might now be experiencing a wage premium, she acknowledged that this was an explanation she considered plausible.